1. Field of the Invention
The subject matter of the present invention involves systems incorporating transmitters and receivers for use in measuring the propagation time of electromagnetic signals. The systems interpret these signals to calculate a location of a receiver, e.g., a position by latitude and longitude, or to determine information about the Earth""s atmospheric conditions. More specifically, the systems typically include satellite-transmitters and earth-based receivers, such as are found in the Global Positioning System, and the invention pertains to improvements in these systems for the purpose of reducing interpretation errors that result from ionospheric delays.
2. Statement of the Problem
A variety of radio positioning systems, such as the Global Positioning System (GPS), are now in use throughout the world. As used in this application, the terms xe2x80x9cGPSxe2x80x9d and xe2x80x9cGlobal Positioning Systemxe2x80x9d are used to include any positioning system that operates by the action of electromagnetic radiation, and not just the specific Global Positioning System that is operated by the United States Department of Defense. The GPS industry grosses more than a billion dollars each year in sales and service by providing reliable, quick, and accurate position measurements. Even so, there are practical limits to the precision of measurement that are obtainable from GPS systems. Traditional GPS surveys can obtain xcx9c2 to 5 mm horizontal and xcx9c4 to 10 mm vertical precision over 1000 km baselines. Extensive literature on the GPS and on GPS software is available.
GPS software calculates position using conventional techniques to determine distance as a function of the velocity of light and time. The calculation of true distance as a function of the velocity of light and time is complicated because the speed of light varies depending upon the content of the ionosphere, the troposphere, and other portions of Earth""s atmosphere. For example, the troposphere contains localized unmodeled water vapor distributions that may affect accuracy. The ionosphere includes a blanket of electrically charged particles at an altitude of about 50 to 1500 km above the Earth. The peak electron density in the ionosphere is in the F2 layer, which exists at a height of about 400 km.
Charged ionospheric particles cause variations in the velocity of light passing through the ionosphere. The delay of GPS signals in the ionosphere is inversely proportional to the square of the carrier wave frequency, and proportional to the total number of electrons that are encountered by the GPS signal along its atmospheric traverse. Uncorrected ionospheric delay introduces significant errors into GPS calculations. Uncorrected delay effects of the ionosphere can range between 1 to 100 meters. Thus, for precise GPS applications (like mm-level surveying) ionospheric effects have to be corrected.
Signal processing techniques have been developed to correct for ionospheric variations in signal propagation time by taking advantage of the two frequencies transmitted by GPS. GPS satellites broadcast carrier signals of different frequencies, namely, L1 at 1.575 GHz and L2 at 1.227 GHz, to permit conventional calculations that eliminate ionospheric errors. Earth-based GPS receivers of better quality collect these dual signals in a conventional manner to eliminate ionospheric errors. A well know linear combination that is referred to in the art as xe2x80x9cLCxe2x80x9d, or xe2x80x9cL3xe2x80x9d is formed to calculate a total propagation delay time that is free of ionospheric delay. A problem exists with this method, however, because these receivers must be dual frequency receivers at the L1 and L2 carrier frequencies and the LC linear combination is by a factor of 3 noisier than the L1 GPS data. This dual frequency requirement increases the cost of the receivers by an approximate factor of ten. Where an advanced single frequency receiver may cost about $1000, a dual frequency receiver costs about $10,000. Additionally, the increase in noise limits the accuracy that is achievable with dual frequency GPS observations.
Various solutions have been implemented in attempts to supply less expensive single frequency receivers with information to correct for ionospheric delays. These solutions typically require some form of modeling. However, all the modeling techniques that are in use fail to correct for rapidly changing or small scale ionospheric phenomena. Present ionospheric correction techniques are therefore not sufficient to permit high quality survey work with single frequency receivers over baselines of 3 km or longer.
The ionosphere may be described as a map of total electron count (TEC). These maps show the integrated number of electrons in the vertical direction as a function of geographic latitude and longitude. TEC is expressed in TEC units (TECU""s). 1 TECU corresponds to 1016 electrons contained in a cylinder aligned along the line of sight with cross section of one square meter. Due to the fact that charged particles in the ionosphere are generated by the Sun and the Earth rotates under the ionosphere, TEC maps are usually represented in a Sun-fixed reference frame where the maps do not rotate but still change with time.
Solutions that permit the use of single frequency receivers include the use of dual frequency receivers to provide ionospheric maps to correct the ionospheric effect on the single frequency receiver data. Dual frequency receivers for this purpose are placed in fixed locations and provide TEC measurements in the directions of all observed GPS satellites above the horizon. These counts are scaled into the vertical direction at the ionospheric transect point. This transect point is assumed to occur at the intercept point between the site-satellite line and a shell at 350 km to 400 km above the Earth""s surface. This shell height is selected because this is the approximate location of maximum electron density in the ionosphere. These scaled observations are interpolated to provide global or regional ionospheric maps depending upon the areal extent of the observation network.
By way of example, global ionospheric TEC maps are generated by the Jet Propulsion Laboratory, the University of Bern, and other sources for publication on the Internet. Regional TEC maps are generated and published on the Internet by the University Corporation for Atmospheric research, and other institutions. These maps have root mean square uncertainties of 2 to 4 TECU""s (this corresponds to 0.32-0.64 cm in delay of the L1 signal), and the resolution of these maps is insufficient to accommodate many localized ionospheric disturbances.
Operationally, a single frequency GPS receiver accesses the TEC map after making a preliminary positional calculation and determining a transect point with the shell. The TEC value at the location of the transect point is read from the map and scaled into the direction of the observed GPS satellite, to correct the positional calculation for ionospheric delay. This kind of correction is referred to in the art as, using the regional or global map and the standard ionospheric correction. In the context of the present application, the network that collects data used for generating the ionospheric maps is defined as the xe2x80x9cmonitoring networkxe2x80x9d and the network that uses the correction (i.e. to improve single frequency positions) is defined as the xe2x80x9cuser networkxe2x80x9d. Unfortunately, this correction is not sufficiently accurate for mm-type GPS work.
Wanninger, xe2x80x9cEnhancing differential GPS using regional ionospheric error models,xe2x80x9d 69 Bulletin Geodesique 283-291 (1995) describes a technique to generate higher resolution ionospheric corrections based on interpolation of the ionospheric delay measured at 3 monitoring sites. The corrections described are computed at the level of double differences. Double differences are differences of the observations of two satellites as seen from two GPS receivers. Double differences are commonly formed because they allow for the highest precision GPS analysis results. The ionospheric corrections are applicable only for satellite pairs and station pairs. Thus, the technique described in Wanninger requires observations of reference satellites and reference stations. The technique also requires that baselines (differences) have to be formed between observations from the dual frequency xe2x80x9cmonitoring networkxe2x80x9d and the single frequency xe2x80x9cuser networkxe2x80x9d. However, since these networks typically use different antennas and hardware, this mixing of data from the networks introduces additional errors.
For the reasons stated above, there exists a need to improve single frequency GPS calculations to be as accurate or better than dual frequency calculations by providing high quality ionospheric correction values to a user network with out the need to provide any of the xe2x80x9cmonitoring networkxe2x80x9d observations, track any specific reference satellites, or require any specific reference sites.
The present invention overcomes the problems outlined above and advances the art by providing a method and apparatus that is used by single frequency GPS receivers to eliminate ionospheric delay effects without the need to provide any of the xe2x80x9cmonitoring networkxe2x80x9d observations, track any specific reference satellites, or require any specific reference sites. This advantage is realized by implementing a new zero differencing technique together with a satellite and measurement epoch-specific ionospheric model based on these zero differences.
A positioning system according to the present invention contains one or more receivers that can be operated in single frequency mode, a plurality of dual frequency receivers, a plurality of satellites or transmitters. The dual frequency receivers receive signals that are broadcast by at least four of the transmitters to provide a plurality of measurements representing ionospheric delay. A central processor connected to the monitoring network is configured to receive data from the dual frequency monitoring receivers. This processor converts the plurality of GPS measurements into ionospheric delay represented as zero differences. The single frequency receiver(s) receive signals that are broadcast by a plurality of the satellite-transmitters to provide a plurality of total delay measurements. The same or a second processor then corrects the total delay measurements for ionospheric delay through the use of the zero differences.
The manner of correction preferably includes fitting the plurality of measurements representing ionospheric delay for each satellite and at each epoch onto a mathematically defined plane surface for the location (longitude and latitude) of the single frequency receiver for the observed satellite at the observation epoch. The value of the mathematically defined plane surface at the longitude and latitude of the single receiver site identifies a value corresponding to ionospheric delay for the specific satellite at the specific epoch.